This invention relates to electrical safety circuits and more particularly to a short-circuit monitoring circuit for a three-phase, variable speed motor, and the supply lines that connect the motor to a constant-current, d.c. link converter for varying the rotational speed.
Electrical short-circuit monitoring arrangements of the above-specified type are particularly significant in pit-mining operations. Due to the danger of overheating and thus possible explosion in the event of short circuits during an otherwise explosion-proof operation in mines, it is necessary to monitor not only the motor, e.g., by means of a motor circuit breaker, but also its supply lines in the potentially explosive areas. The monitoring itself should take place outside this area because otherwise the monitoring system itself would also have to be designed also in an explosion-proof fashion. In addition, monitoring the supply lines is even more important than monitoring the motor since the latter has a thermal overload protection device. Underground mining therefore requires that a power line carrying a short-circuit current has to be separated from the supply system within 100 ms. Generally, this objective is attained in constant-current d.c.-link converters of the above type by monitoring the magnetic flux of the connected motor which, as a rule, is integral to the system. Said monitoring, however, only begins to operate at frequencies exceeding 5-7 Hz.
This corresponds to a motor rpm of approximately 10% of the rated rpm in 50-Hz systems. In the range between 0 and 10% therefore the lines between the converter and the motor, and to the motor itself, remain unmonitored for short-circuits. Neither can a power measurement be used as a monitoring criterion, since due to the always prevailing current-limiting circuit in the converter control circuit, the monitoring would not be able to differentiate whether the excess current was due to overload or a short-circuit.
It is the object of this invention to develop a fully functional short-circuit monitoring circuit that operates in the 0 to 10% range of the no-load rpm, as well as in the rest of the range.